Why do political constituencies delegate decision power to representative assemblies? And how is the size of such assemblies determined? We analyze these questions of constitutional design in a model with voters learning their preferred alternative only after engaging in costly information gathering. We show that there is an optimal assembly size that would be chosen at a constitutional stage. This implies a relationship between assembly and constituency size. We then compared this relationship to actual data. Fitting a single parameter to the data, we show that our model can explain the actual relationship between assembly and constituency size quite well.